Thanks a lot for your answer Ashey ! (and sorry for the delay, I've been quite busy with work).
I don't know why there are so many images. I thought I followed the tutorial properly, to create a tile set and to create animation.
My game having 7000+ images is wrong. I must have done something wrong so I'll check that asap.
But I would like to confirm something first to understand how the system works.
In the plateformer tutorial, you teach us to create animated sprites.
1) Create a new Sprite
2) Add animation
3) Import images
4) Repeat for each different animation
So my game uses this system with around 20 sprites, each having around 10 unique animations with around 10-15 images in each animation.
So my question is : Is this still the correct way to create animated sprites ? Each animation is supposed to create one animation sprite when exported ?
Also, you said "it looks like you're using animation frames as tilemap tiles - that's going to use up a lot of images", well that is true. Why did I do that ? Simply because I followed the tutorial for the plateformer game which shows how to create tiles by the method I used : Create a sprite, import all tiles inside and change animation speed to 0. I think when I started using Construct 2, the tilemap object did not exist thought (1 or 2 years ago I think).
I'll update my project with the new tilemap object then. Hope it'll lower the number of image generated.
But I really wonder how so many images can be created indeed. I really thought I followed the tutorial to the letter.
Thanks again for your help !