You have two options here.
- If you have sequentially numbered still images you can import as an image sequence witch is essential a movie by selecting any single image in the sequence and checking the image sequence box
- If you have any folder with images of any name you can import all of the images as still images by selecting multiple images and not checking the image sequence box.
With option 1 when you drag the imported image sequence into a timeline or create a new comp from the image sequence you will essentially have a video. The default frame rate for image sequences is 30 FPS, which may not correspond to your composition's frame rate so you may have to change the interpretation, or if you are like me, change the preferences to match your most common frame rate. You treat the image sequence just as you would any other video. You can animate the Liquify effect so that it only effects a certain part of the video.
With option 2 you can create a 'movie' by selecting all of the still images imported into your project panel and either dragging them to the new composition icon or right clicking and select new composition from selection, or drag them to an existing timeline. With the first two procedures you will be offered to import as layers or create a new comp from each image. Select layers, then set an out point for all layers at the number of frames you want for each image then, with all layers selected, use the Keyframe Assistant to sequence layers. You can then apply the liquify effect to the desired layers. HOWEVER, it will be much more difficult to get the liquify effect to work and animate the way I think you want it to with option 2.
The biggest problem we are having trying to help you is that it is not very clear what you are trying to do or what kind of image sequence you are working with. If you are very new to After Effects be warned, there is a lot to learn. You have to have a good understanding of how compositing works, how key framing works, how video works, what the various types of compression are and about 100 more topics before you can just dive in a do some effects. Give us a better description of your problem and we may be able to give you better suggestions.