Each social media outlet has its own image sizing guidelines. Although it's possible to upload nearly any size of image below 5 MB to any social network, uploading images without cropping them to suggested specifications reduces the likelihood that your images will look good in their placeholders. I found a handy sizing guide on Mediabistro which you can access and view here. When uploading new images, take a few extra moments to properly size your images. You'll reduce the likelihood that your images look pixelated, fuzzy, or are cropped sub-optimally.
Pixlr
Don't have Adobe Photoshop? No problem, I love using Pixlr to make many crops and adjustments. The free online application allows users to quickly and easily crop photos to social media outlet requested dimensions. Adjustments take only a few minutes.