Based on this FAQ:
There is a 10 MB limit per image, but based on some previous threads, 1 MB appears to be closer to the optimal size for uploading (takes less time to complete the upload).
However, there does not appear to be any specific guidance on how many images to upload to a single profile. I suspect it has more to do with the profile, the content of the images, how they pertain to genealogical data for that profile, and whether or not you own the copyright to the information. If you believe this is the most appropriate way to get the data into the profile, then I would proceed with uploading the information as images. But keep in mind that the text in an image can't be searched on, and you may want to try to limit uploading an image only once that applies to more than one profile and then referencing it on multiple profiles. If you believe the text of a source document is fairly important, I would consider entering it in as text and not as an image. If you have a large number of sources that apply to multiple profiles, you can consider a Free Space page for the images and then reference them from that page.
Also, if the image is available online and you can point to it, I would also consider leaving it on the source site if at all possible. There are some unique copyright scenarios with certain images that can create problems, and we'd prefer to keep WikiTree from getting into situations where the image can be construed as copyrighted and we end up being asked to take down the image.
I think that's all I can come up with. Maybe others have some thoughts?