There's no special limit on the plan icon image. You should check its path within the sh3f file. Be sure to modify a separate sh3f file and reimport it at each change, because modifying the sh3f saved in your profile folder won't work when Sweet Home 3D runs.
I'll be also happy to know if the new feature in Sweet Home 3D 5.5 matches what you look for
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Emmanuel Puybaret, Sweet Home 3D creator
