At the moment this is the best image resizing app for iPad that I have found.
Batchresizer2 has a very cluttered user interface, but it does what it should very well so I want to recommend it anyway.
The app can resize images almost any way you like. You can set the desired maximum height or width or both. You can convert between image formats. You can set compression levels for jpg:s. You can save presets for conversions you make often. You can add borders and shadings. You can have images automatically cropped to a square after resizing.
The app can also be used for turning images 90 degree at a time in any direction, but not in batch mode. It has to be done image by image.
This app is free with ads. You can pay about 2 USD to get rid of them. I think it is good enough to pay for, especially since there aren’t a lot of other apps doing the same thing. It would benefit from getting a cleaner interface with less unnecessary colors and details, but it is what it is.