Currently there is not a sure-fire way to do it, no. The only way that it could be done is to use Sims 3 Package Extractor (s3pe) to replace the current build music in the FullBuild1.package file. If this was all you had to do it'd be easy. However in The Sims 3, EA thought it'd be a grand idea to package the music in it's own proprietary format ".snr/.sns". This is also known as the EA Layer 3 audio format. They also used this format in Command and Conquer 3. To get something encoded in this format is the problem. There are no known converters that will encode into it. I myself, have gotten ffmpeg to convert out of this format into mp3 for some files, but not the other way around. I did not put more effort into it after that and moved on to more enticing projects.
Unlike in the Sims 2, each save file exists in its own, separate universe.
In the Sims 2, time did not pass for sims you weren't playing. However, in the Sims 3, the "Story Progression" means that no matter which household you play, time will pass for all families in the neighborhood.
The one thing that is shared between neighborhoods is the Family bins -- that is to say, both you and your fiancee should have each other's sim in their neighborhood bin. If you then move this sim into the neighborhood (even if you don't change the active household), you will be able to find, meet, seduce, and eventually marry each other's sim.
However, because you have separate save files, marrying in one neighborhood won't simultaneously marry the sims in the second neighborhood.
It is possible to simply play multiple active families in the same neighborhood, but since every household ages at the same rate in The Sims 3, this will probably not be a preferable way to play (not to mention losing wants every time you switch active households).
Best Answer
I wasn't able to find anything on putting in 'custom pictures', but I was able to find how to add custom music.
You can add custom music to all the sims games (as outlined here) but sinced you asked for just The Sims 3 and The Sims 4 that's all that I will put here.
Just a couple of notes; when adding custom music, it just appears under radio stations and you cannot change the name of the radio station it appears under (this applies for all versions of the sims), also all the music has to be in .mp3 format.
The Sims 3
To add music to the "Custom Music" radio station in The Sims 3:
My Documents\Electronic Arts\The Sims 3)The Sims 4
To copy music to a genre specific radio station in The Sims 4:
My Documents\Electronic Arts\The Sims 4)