Well, let me be direct: It would be useful if you actually read the AE help. Your awkward use of terminology alone is a indicator that you don't understand some of the basic principles like footage interpretation vs. comp settings for framerates, timecode settings or dealing with image sequences. You cannot find tutorials because you are not using the right words. There is probably nothing "quite specific" at all going on, you are just muddling around because you don't know these things. Beyond that we can't know anything. You have not provided any info e.g. about the actual naming/ numbering of your images, how you import them, system info, version info for AE and so on.
Changing the frame rate of a composition does not change the frame rate of the footage you put in the comp. If you have a 30 second clip that was shot at 60fps real time is 30 seconds. If you put that clip in a comp that is only 10 frames per second the clip will still take 30 seconds to play back it will just not be as smooth because you'll only see about one frame in 6.
The default frame for importing image sequences is set in your preferences panel. It is 30 fps. 100 frames @ 30 fps is 3 minutes and 9/30 of a second or 3:09 @ 30 fps.
If you put that image sequence in a comp that's 25 fps then the last visible frame will be 3:07 and the clip end at 3:08(no more image) This makes perfect sense. If you look at the info to the right of the thumbnail for your image sequence when you select it in the project panel you'll see that it is interpreted at 30 fps. You can change that by selecting the footage and then right clicking or going to File>Interpret Footage > Main and change the footage to 25 fps and your problem will be solved.
Now let's talk about frame numbering. Image sequences should be in their own separate folder. If you have 100 frames then the frame numbers should have 3 digits and exactly the same name. myFootage_1.jpg for the first image of a 100 frame sequence isn't going to work. The frames will be out of order. You should use myFootage_001.jpg for the first image and all other images should be exactly the same until you get to 100. The most likely cause of your problems with the import are directly related to the file names.
I hope this helps. There are basic things that apply to all motion pictures and video. The Search Help field at the top right corner of AE is your best friend when you come across something that is confusing you. It will point you to some pretty decent community resources. Video is hard. You have to understand formats, compression, standards, frame rates, rendering, delivery formats, production formats, and a bunch of other things like pixel aspect ratios and interlacing to successfully work with video. AE requires a much bigger foundation in animation and compositing techniques just to get some of the basics done reliably. Study and learn.
Dorottya - read Rick's reply carefully!
Now , here's an example on how you can adjust your 100 pictures into a 4 sec sequence.
Thank you so much. Your explanation was very helpful it solved all my problem with Ae. Simple and straight to the point. The way I needed someone to explain it to me. I really do appreciate it.