Use a netcat pull from the busybox install (if netcat is available).
Copy desktop.jpg from the device to your pc with the netcat/nc method:
On your pc, disable temporarily (or reconfigure if possible) any firewall and run
netcat -l -p 10000 > desktop.jpg
and on the device
busybox nc A.B.C.D -p 10000 < desktop.jpg
where you need to replace A.B.C.D with the IP address of your pc. As soon as the transfer was successful, the netcat process on your pc should stop automatically. If not, something could have gone wrong and you can stop it with Ctrl+C (compare source and destination checksums to verify).
For the other direction, just exchange < and > on both sides. Make first a backup of the original desktop.jpg (cp desktop.jpg desktop_orig.jpg).
Use a netcat pull from the busybox install (if netcat is available).