Normally if you want to edit some photos on windows you use photoshop, but since we don't have photoshop on linux, we must search for an alternative.
On linux we have software called GIMP as image editing tool, actually GIMP also available on windows too.
GIMP is a raster type graphics editor just like photoshop, so it's should be good enough for photo editing/retouching.
The way you operate GIMP is different from photoshop, so in this case you have to learn how to use GIMP.
Alright, to install GIMP on lubuntu all you have to do is open the terminal console and then type the following command.
sudo apt-get install gimp
Make sure your internet works, once installed, you can open GIMP from the start menu > Graphics > GIMP Image Editor.
Basically what i usually do with photoshop is selecting, cropping, and resize image and i could do the same using GIMP.
In my opinion GIMP is a very good alternative for photoshop, specially if you're working on linux system.
All you have to do now is learn how to use it, i think GIMP has the same capability as photoshop, we just need to know how to use it.
~ cheers ~