The best practice is to resize an image in a photo editor before you upload it. Use Photoshop or see Resizing Images Using Paint. If you use the same image at two different sizes, both sizes should be uploaded.
You can change the size of an image in the text element, but that does not reduce the size of the image - it only changes its display size.
It is possible to permanently resize an image using the edit feature for images inside CommonSpot.
You may need to clear the page cache - Actions > Clear and Update Page Cache (bottom of list).
If the size is too big, you should upload a new version of the image after you have resized it in Photoshop.
Avoid this issue by uploading multiple images (even if you are only uploading a single image).