Graphic wont move smoothly on object

Here is the reason and workaround. Glass Object is located in front of Screen and blocked access to screen pixels  for selecting decal, but we are able to use rotate and scale handles.

Workaround

1. Hide Glass object before moving image

Also if we have only Glass and Screen objects we can add distances between them. Then after changing angel for camera (see middle tablet object) we can access to image and move it.

Solved the issue in my case

You placed the graphic as decal to answer the first question, now tell Dimension to export the UV map to get a Photoshop file you can fine tune by hand

If you want this design to cover more than one face of your cube I'd go with a fill instead of decal because it's not going to then have the helpful Adobe Ai trying to correct your design

Could send a screenshot, so we can spot if anything seems odd on your object and graphics

Also did you add it as a decal or fill?
Because sometimes, one might be better than the other depending on circumstances

Sometimes also, you can try to generate UVs

that's sound weird but somtimes if the initial object (cube) have been transformed or tweaked in any way, the graphics applied can get a little funky. The generate UV function, re-compute the UV from the actual object.

See if that helps!