You can use PhotoPlog found
here. I don't think the lite version allows for watermarking but the pro version does. I believe it's image based as well so you could make it any size you want ; )
The pro version does cost but it may suit your needs. I have no affiliation with Calorie just thought it might help you. I used the lite version on my board for awhile and it worked great. I was gonna go pro but ended up going in a different direction so I didn't need it. You can demo the pro and lite versions on his site. I believe most of the images on his pro demo are watermarked.
Hope that helps!