Can't add background image to Smartsheet form

I've easily added background images to Smartsheet forms in the past, but now I'm having the following happen:

• I open the Edit Form window and click on Edit background

• I upload the image (which is less than 5MB) and the image appears successfully in the preview

• I click on Apply in the edit background window and the form closes, the relevant sheet opens and the background on the form is not updated

Does anyone have any thoughts please on what's happening?