Getting image sizes consistent through plugins?

I’m using a theme that has an image slider on the front page, and I’m having trouble keeping the image sizes and proportions consistent. WordPress will automatically scale down an image, at least on one axis, but sometimes the only image I can get is too small in one direction yet ok or large on the other. This results in a very mismatched looking slider and the body of the page below moving up and down in response to the height of the image above.

I was hoping to find a plugin that would give me a consistent image size each time, damn the original proportions. If there’s not enough data to fill the image, it could use a blown-up darkened or blurry copy of the image as a backdrop. (I’ve seen this done on news sites, but I don’t know if there’s a WordPress plugin with that functionality.)

Not sure I’m posting this in the right category, but the Plugins and Hacks section seems to be for questions about specific plugins.