Hello Paul thanks for posting in the Lucid Community! 

If you are unable to upload images it is possible your storage is full. You can check how full your storage is in your account settings. Images take up the most space in your account. If you are running out of room it is recommended that you clear out as many images as you can from both your documents and your Image Manager. You can access your Image Manager by clicking the image icon next to the magnifying glass above your shape library. 

For more information on how to upload search for and manage your images please see our Shape libraries in Lucidchartand let me know if you have any questions about the set up or intended functionality.

If your storage is not full can you try inserting the images from a private or incognito window and see if it works from there?