Deziner, if you click on the attachment button whilst modifying the post you'll see the restrictions on images..
If you have Windows 10 you can use the picture app to resize your images. Once the image is up in the app if you right click you'll see an option to resize. I typically use the lowest setting but there is still a restriction on number so you can only add 4 pics once they are within the limits below.
Allowed file types: doc, gif, jpg, mpg, pdf, png, txt, zip, gpx, pvm Restrictions: 4 per post, maximum total size 6000KB, maximum individual size 3000KB