Fixed PNG that doesn't open in treesheets and with IDAT error in GIMP

The image fixed in this commit was causing Treesheets to spit out the following error when starting up:
```
Warning: IDAT: invalid distance too far back
Error: Couldn't load a PNG image - file is corrupted or not enough memory.
Error: Failed to load image from file "./images/nuvola/dropdown/bookcase.png".
```
The image file wouldn't load in the viewer program I tried to check it in, and would load up with a similar error about IDAT and an invalid distance before displaying when I would load it in GIMP. This occurred on an up-to-date Archlinux box.

To fix this issue I simply re-exported the file in GIMP, and Treesheets now loads happily. I went through and checked all other image files in the `images` directory, and everything else was fine.