Q3A is able to use textures in tga OR jpg format (although it'll always refer to tga files). So if you can't find a tga file but a jpg file with the stated name, cut and paste the jpg file.
If you see several messages of this kind, I would recommend to make a condump into a file (e.g. '/condump texerrors.txt').
2. Unpack the pk3 with WinZip or another zip-packer (keeping the folder structure used in the pk3!).
3. Locate the .tga files stated in the error messages.
4. Cut and paste them to the correct folder(s).
5. Pack the whole thing again. Rename the packed file from xxx.zip to xxx.pk3, and here you go...
Of course this will only work, when the 'missing' textures are included in the pk3 ;) ...and it seems they are, in this case.