Please see this: Create custom forms - Power Automate | Microsoft Learn
"Store the URL in a variable earlier in the flow, escape the percentage characters , and then use that variable in the URL property. The preview won't show the image, but it will be shown during runtime."
Please check if it works during runtime, also worth testing if storing the file-path in a variable earlier in the flow which you can use in the URL field in the custom form designer.
If you are unsure how to do this, just use the Set variable action, give the variable a suitable name, and type the file-path in the value field.