Textures are not appearing in the LightBox browser in ZBrush 4r6.
Not apear or just appear in black icon?
Is is that may be is the file extension or name. It´s kind of tricky but if your file does not have the .jpeg extension labeled will not be showed in the lightbox. So to fix that in windows just need to rename your image as you want but .jpeg included
You need to upgrade to P2:
http://www.zbrushcentral.com/showthread.php?178981-Questions-amp-Troubleshooting-for-ZBrush-4R6-read-first-post-for-P2-Patch-instruction
Upgrade - ZBrush won’t import JPG files either.
A: This issue has now been resolved by a patch. Run ZBrush 4R6’s auto-update (or the ZUpgrader application in your 4R6 folder) to download and install the patch. DO NOT run the one in your ZBrush 4R5 folder. After updating, your ZBrush will say “ZBrush 4R6 P2” in the upper left corner. If you have not yet upgraded to 4R6, you’ll receive the P2 version automatically as part of your upgrade to 4R6 and will not have to update after installing 4R6.