While Lemmy does not support image or video "albums" natively, you can add as many images and/or videos as you want to the post's body (within reason).
It's better to do this then create multiple posts if you have multiple images to post. Simply upload the main one as the URL and the rest in the body.
Edit (thanks to @ppptan):
When doing this, it may be better to do [![][1]][1], then later in the post do [1]: {image url}, replacing 1 with different numbers for each image, in accordance with this. Additionally it can be nice to include spacing between each image to keep it neat and tidy!
Posting the images in this way also allows a user to more easily open the image to view it fully.
Example
Adding this to the body of your post (as an example):
[![][1]][1]
[![][2]][2]
[![][3]][3]
[1]: https://lemmynsfw.com/pictrs/image/b4281970-bad0-44d7-9aaa-7fefaf411212.webp
[2]: https://lemmynsfw.com/pictrs/image/e732ea1e-48fa-45e2-bbd4-966982381a23.webp
[3]: https://files.catbox.moe/jg85na.jpeg
would give you something like this (You can left click them 🤯):
Please also mention that the post contains multiple media in the Title of your post, so that people know to open up the actual post to view the rest!