It seems that your software doesn’t support given file format or file you’re trying to load it is too big. You should scale down hdri image using Adobe Photoshop. If you don’t have Photoshop you can use some other software like free Picturenaut (Windows only) or Photomatix Pro trial for free – it is not time limited trial, so you can leave it on your computer. Photomatix is available for PC and for Mac.
Steps to scale down hdri or to change format in Photomatix:
- Got to File > Open and choose your large hdri image, wait until it loads
- Go to Utilities > Resize and enter safe number (for instance for Maxwell Render max it is15000 x 7500px)
- File > Save As and choose your format (.exr or .hdr)
- Recommended compression for .exr is PIZ