You can’t but if you add a screenGUI on a part and then add an imageLabel, then copy and paste the decal id in the imageLabeltheb it should be what you want. Hope this helps!
Copy and paste the decalId into the “Image” property of an “ImageLabel” or “ImageButton” instance, the field will automatically convert the decalId into its corresponding imageId.
OH WAIT DIDNT SEE UR TRYING TO CONVERT SOMEONES ENTERED IMAGE ID idk it should work how you have it
OLD:
You have to upload it in the asset manager
Go view > make sure asset manager is selected > in asset manager double click on images >
click this >>>>
select the file and upload then you can right click on the recently upload icon and should be able to copy id