There are no "portrait pacts", there are "portrait packs", which are just folders of PNG(mainly these), JPEG, or WEBP images. They often have descriptive folder and file names, usually this is according to the tagging system established by the Improved Random Portraits mod. The vanilla game has random portraits, but it's a very simple system. Most(if not all) of the portrait packs are behind in versions, but they will work mostly right as is and can easily be modified to work correctly. The reason for this is that most packs were created prior to version 1.0 of Strive, when a couple of the elf races were renamed. That is explained here: https://itch.io/t/973991/transition-mod-for-using-v0525-saves-on-v10a
Portrait packs are not really installed. They usually have one or both of the "portraits" or "bodies" folders inside them and you simply put them in the matching folders in the user data folder. The location of these folders is pretty much the same as the "mods" folder, so I will simply refer you to the in-game guide to mods which explains where it is and how to access it through the game. The modding guide is in the "Mods" section of the main menu and press the "Help" button in the top right corner.
I don't have recommendations about portrait packs, as I haven't bothered keeping up with the characteristics of each and whether they still have active links. Often the older packs have active links posted near the end of their comments. While you can use as many of the portrait packs as you want, the manual image selection interface for slaves will crash if you have too many images, so you will have to rely on the random portrait button from the mod.
Mods are can be quite the tricky thing for Strive. The modding guide covers the basics quite thoroughly, except if you have MacOS that portion of the guide is not entirely accurate (https://itch.io/post/2847934). Some mods have their own install instructions, and it is important to pay attention to those. It is not always possible to identify which mods are compatible with each other without either testing it or reading the code of both of them. Some mods must match the game version exactly and other mods have remained stable without updates for a long time. Some mod authors have taken the time to test compatibility with other common mods.
My recommendations for mods will depend on how much of the vanilla game you have played and how much new stuff you want. I have not tried that many of the mods so these are based on mod authors and users. If you want confirmation that multiple mods are compatible, I can check their code but I don't plan to do that preemptively. Otherwise, you can simply try it, and if it crashes you can remove mods or in the worse case copy the contents of the "backup" folder over the "files" folder (that's how the reset works). Though the mod system will not know if you manually alter the files to add or remove mods, but the interface selection is just for user convenience.
If you are just starting and want a proper base for vanilla play, then Bugfix V5 and Improved Random Portraits mod (by Zepeteus).
If you want a little more variety, then add Leo's mod v1.7 and Expanded Sex mod v1.3e (distributed with Chastity Belt mod). If you want to try the Chastity Belt mod, then you will need to forgo Leo's mod, as those two are not compatible. There are lots of little mods that change various parts of the game, which may or may not be compatible.
If you want a lot more variety, then try Ralph's mod and Improved Random Portraits mod. But note that it is quite different from most other mods and is downloaded with a pre-modded copy of version 0.5.25 of Strive. It will not be compatible with almost every other mod.
Note, the MAD mod is very outdated and will not work.