Hello @ShirliM !
Maybe you exceeding your account storage limits (which can be viewed under your account settings page) or that Shopify may be rejecting the file because it is too large. If you're not exceeding your account storage limits try saving the file as a PNG file and then use this website: http://kraken.io - it's a free online tool that will let you reduce your file size by up to 80% without affecting the quality.
Hope this helps!
Hyde here from Shopify.
I believe @DigitalArtisans has suggested what the issue may be pretty well. To elaborate: you can upload images on the Files page of your Shopify admin. Are you uploading according to our file requirements?
What file formats you may use depends somewhat on the plan you are on:
What plan are you on? And what formats and sizes are you trying to upload? The more context you can provide me, the better I can help you!
All the best, Hyde.