Minecraft – Can’t find screenshotted photos in Minecraft Technic Launcher

Screenshots taken in Minecraft on MacOS (using the in-game function, by pressing Fn+F2), are stored in the directory ~/Library/Application Support/minecraft/screenshots.

The source additionally provides the following as an alternative for finding your screenshots:

You can also access the screenshots folder by going to "Resource Packs" in the settings menu, clicking on "Open resource pack folder", and going up one level.

In your case, using the Technic Launcher, they might also be stored in the folder of the mod you were using when you took the screenshots, meaning in ~/Library/Application Support/techniclauncher/modfolder (see here).