If you've uploaded images that are too small, or need to be replaced for some other reason, there are three ways you can fix the situation.
1) Delete the existing work and replace it with higher resolution image sets. This is easiest, and if you don't have any transcriptions for the work definitely the way to go.
2) Replace individual images in the work with higher resolution images. To do this you'd log in as the project owner, then go to the work's "pages" tab. i.e.:
https://fromthepage.com/
Then you'd click on the "settings" action to get to a page like this:https://fromthepage.com/
You can replace the image with a larger one in the Page Image field.
This approach would be tedious for large works, but it would maintain the transcriptions and the history of the work. If a work has a lot of transcribed material it might be the best thing to do, perhaps only for pages that haven't been transcribed. (After all, if it is transcribed than the resolution was "good enough".)
3) You could also upload a higher resolution set of work images to a new work (same as #1) and then manually copy and paste the transcriptions to the pages that were already transcribed. I might do this for a work that is mostly untranscribed, but has a handful of pages you want to preserve. Be careful that you don't upset the transcriber, however, as this approach will lose their attribution in FromThePage (although you might be able to ask the transcriber to do the copy and paste -- perilous, but if you have a good relationship it might work.)