I love Aldiko. I read all of the Sherlock Holmes stories on it this summer on my mobile phone. Bump the text size up a bit and it's perfectly readable.
Today I tried to change the covers of some of the books in my collection, something I've done before, but I couldn't remember how to do it.
I finally figured out how I'd done it. This time I'm writing it down so I can remember.
Download a cover image. In Aldiko, long press on a book on the bookshelf. Select Details, long press and release on the book image in the pane that appears. Select Gallery, go to Downloads and choose the cover image. Select or crop as necessary.
Using the file browser to find the image didn't seem to work for me, although it may have been a glitch.