is there an option or a plugin that resizes an image depending on it's orientation?
Reason for this question:
Im building an image grid and it would look even better when all images would have the same width.
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If you need the images to be the same width but varying heights (ie not 300 x 300 but 300 x anything) then in settings -> media add 9999 to the height of whichever image size you want to use.
You can also define your own custom thumbnail using the same principle: http://codex.wordpress.org/Function_Reference/add_image_size
[EDIT] As @brasofilo suggests, you can also use 0.