You can actually just set a %width on images, and the browser will resize them itself, so you can just use the one same image in all situations. (Older PC browsers tended not to render the images so well, but not sure how they handle this now, but I assume it's OK, as this is a common approach.)
The downside is that you are asking people on small screen (mobiles etc.) to download images that are larger than needed. So some clever people out there are working out ways around this. It's still a developing area (the perfect solution hasn't arrived yet, I'm pretty sure). Here are some useful links, though:
All the same, if someone is viewing a gallery on their mobile, they've got to make a decision whether it's worth the downloads or better to wait till they get home. I'm not so worried about the image issue, but it's still worth finding a solution.