The perfect size photo for the profile cover image is 745 pixels x 275 pixels. uploading an image that size prevents it being distorted to fit. There is a great free image resizing/cropping tool online at :

https://online-image-editor.com (this link opens a new window)

Scroll down for a guide to using the online image editor

Go to https://online-image-editor.com And click 'Upload an Image'
Choose a pic from your pc/ipad etc
Click Resize
Do the above!

the image is now resized to 745 pixels wide, perfect for our cover pic, but still too High

Click Crop
This keeps our perfect width we created, but we can now crop the image down to 275 pixels, the correct height for our cover photo. The red box that appears can be slid up or down as below.
Once you have selected the crop area you want click apply
Now you can see your finished image, note the image goes off the screen here but trust that it is 745x275. Click SAVE and save local to save it to your device. You can then set it as your cover photo on your profile.