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Once I came across the forum port where the man had an issue with uploading images, because their size was more than 2 MB
Depending on the method you choose for image upload you can specify the dimensions of your images.
There is the application - Store Manager for Magento. It allows to resize images on a fly right in the process of import, so you will not need to re-size them manually.
In case, you are interested, here is 2 weeks free version of the application available for download -
Hope it was helpful.