Thanks for the comments.
I do usually use the highest resolutions I can to start with. These stickers were from digital photos I took with only a 1.5 Megapixel camera some time ago, but it was still high enough that I reduced the final prints. Just about any digital camera will take good images to use for decals on 1/25 models. Do be careful of downloaded images because most are low resolution. But, for 1/25 decals it doesn't usually take a huge image.
I printed to the decal sheet at about 800 dpi which is approximately half the resolution of my Epson printer. I find 800 dpi a good compromise between max photo resolution and a readable print on the lower quality decal paper. I'd try doing a good photo quality print on whatever printer you have. The biggest worry is too much ink that would run and lose detail.
All the decals on these doors were printed on white decal paper as no ink jet can print white. The only trouble is you have to cut the decals close to the image. You can see some oval ones I had trouble getting smooth cuts on because they are so small. If I get ambitious I might try some others and apply on top. I always make 3 copies of all decals on the sheet so I have some to make repairs with.
I use Photoshop for all my image editing and resizing, but there are many reasonably priced alternatives.
This was pretty light, so feel free to ask me anything as it comes up.