If you are looking to create a design for someone and you want to make sure the cube is not going to be significantly more challenging to solve than solving a normal cube, then there are a few things you need to consider:

Solving the custom cube will be a lot easier if there is a unique colour around the edge of the picture on each side or a consistent colour theme, this is even better if it follows the standard cube colour scheme:

This is the standard colour scheme, it is actually quite funny watching how speedcubes react to minor changes to this, swapping two opposite colours will really slow down some fast speedcubers, while on others it will have no effect at all.

So to make a simple design, we are going to take images and put a unique colour outline around the image and upload it following the standard colour scheme.

For this guide I will be using Google Drawings but if you prefer another drawing or image editing program then I suspect it won't be too different.

Firstly, in the top bar, go to file - page setup

Then use the drop down menu and select custom

Then with the dropdown that says Centimetres (yours may say Inches by default), dropdown and select 'Pixels', then change the Pixels numbers to 1000x1000 and click Apply

Next you need to find an image you want to use and crop it so that it is square:

Here is a picture of a sheep that I found in Llanfairpwllgwyngyllgogerychwyrndroboollllantysiliogogogogoch, as the background is clearly green already, there is little need for a green outline but it does still need cropping, this image would have to go on the Front side of the cube.

Now I have cropped the image and enlarged it to cover the entire drawing area, this image is now ready to be uploaded.

Now before we proceed, it is worth considering what your image would actually look like on a good speedcube, good speedcubes have Florian holes which help with corner cutting and prevent the cube from locking up, this is roughly what that sheep might look like on a 2x2:

For 3x3s, I am able to request printing on cubes without Florian holes if you would prefer a clearer image, however, the cube will be much stiffer to turn, if that doesn't bother you then just request that at checkout.

Here is roughly what the sheep will look like on a 3x3 speedcube: 

To save time, I recommend going to file - make a copy, this creates a new copy with the same settings, you can then delete the images from the new copy but retain the same sizing settings.

There is some orange on the robin here but it certainly isn't enough to take away the need for an orange background if you want the cube to be easy to solve, I am naming it Left because Orange goes on the left, as per the cube net.

Now I will resize the image to just show the robin, ideally in the middle.

Now I will put orange blocks around the sides, you can do this by selecting the shape tool underneath 'Arrange', choose 'Shapes' and then select the Rectangle.

Enlarge it to cover the entire top area, set the colour to orange, then change the border colour to transparent, then copy it for the bottom and copy and rotate for the left and right sides of the image.

There is a lot of blue on this image of Barcelona so I will put it at the back but also add the blue outlines too.

Once again, crop the relevant parts out of the image and enlarge to fit the square (I am not sure what the relevant part is here though), add blue rectangles around the sides.

This picture of snow in June (taken near the top of Ben Nevis) is rather white so should probably go on the top or Up face, this time it is so clearly white that it probably isn't necessary to add outlines.

Here I have a picture of the inside of an old steam train, the seating area is clearly red but I will still add the red background, it is worth mentioning here that if the bars around the side cover up too much of the image, then you can just shrink the image underneath slightly.

And finally for the Yellow side, which goes at the bottom, we have bananas, although most of them are looking a bit green (except for the one in the middle which you can buy from my main website here).

Once again, I made the image a square, added rectangles around the edges and made them yellow.

And here we have a completely pointless printed cube design, I am sure you can design something better than what I have just done. The reason these designs are easier is because they follow a regular colour scheme so differentiating between the pieces is a lot easier, you still have to make sure the centres are rotated correctly though, this still increases the number of possible combinations on the cube significantly (in fact, it multiplies it by 1024) and requires an extra algorithm to fix.

Obviously If you want your cube to be really difficult then just ignore all this advice and find very similar photos for each side with no distinct colour pattern, just make sure they are 1000x1000 pixel square images for all 6 sides.