PS. If you want something that'll resize pics quickly, download and install this -
https://www.irfanview.com/64bit.htm ***
- Open a pic with it.
- Press Ctrl+R keys, type 900 in width or height box (whichever is showing the largest number) click OK.
- Go to File menu > Save As - give it an appropriate name - click Save.
- That's it - you now have a 900 wide (or high) pic ready to upload.
*** In above link, scroll down to foot of page and click on the link I've highlighted below. That's the one you want if you've got a 64-bit computer (you almost certainly will unless it's an old one).