Hello! I need some help.
I was looking for some painting replacement for the sketchbook, I needed to change the sketches for the story I am trying to make. But it seems like there is no mod for it out there...
Do you have any idea how can I make it?
I've never done anything like it, but it is quite an important part of the story... so, can someone give me some lights on the way to proceed?

You have to replace the images. They are in FullBuild_p18.package (in the UL installation dir).
At a quick glance, they seem named following this format:
sketch_<PaintingSkillLevel>_<SketchNumber>_<InstanceId>
It means that Sims can only draw sketches of level <= their painting skill.

You can replace them all or, to be quicker, use a Sim with level 0 in the painting skill and just replace those of level 0.
You can only replace one or a few and try your luck in game until you get one of those you replaced.

Start S3PE, put "sketch" as name filter and then open FullBuild_p18.
Select and right click on the _IMG resources you are interested in and extract them into a new package
that you'll put into the Mods/Packages folder (name it "SketchesDefaultReplacement" or something like that).
Then open this newly created package, extract the images as files, edit them with a graphics editor that supports
the DDS format (when you save it keep the same options of the original image - DXT 1 compression and mipmaps)
and re-import them in the package of your mod.

Note: extracting the resources into a new package instead of directly extracting them as files it's an extra step
but may help to avoid confusion.

Thank you so much!! I'll try it and see if I can make it
Have a nice day

If you want, you can also override PaintingSkillDB (it's an xml in GameplayData.package, in the TS3
installation directory of course) to remove all but one sketch so you are sure that only the one you
replaced will be selected in game.

By the way, this xml can probably be used to add custom paintings/sketches (even if, as always,
it would be better to use a script to add new stuff for shareability sake).