Guillaume Gécé, modifying "Photo" plugin is complex custom code modification. If you are not a developer, I don't recommend changing anything. Leave "Photo" plugin as it is now.
For your cartoons, I suggest creating separate custom page where you as site admin will post needed images manually using simple <html>.
1. Upload your image using upload form under admin panel>>appearence>>edit theme>>graphics
2. Copy full URL of the upload image
3. Create a new custom page in admin panel>>pages&menus>>pages&menus>>add new item.
4. For the "Page Content" box use your <html>. Here is a simple one.
<html>
<img src="http://sitename.com/ow_userfiles/themes/theme_image_6.jpg" alt="Title goes here" target="_blank>
</html>
Where "
sitename.com/ow_userfiles/themes/theme_image_6.jpg" is URL you have copied earlier.
5. Save changes. Go to your site. You will get a new menu item that will open in a new page with your image.
You can upload more images and use the same .html format for them as well, just keep adding new lines with<img src...>.
You can play around with your .html code. Add several columns for your images, set needed width and height. You will find a lot of tutorials if you google and will get hold of .html easily, while modifying .php code of the "Photo" plugin is not that easy.
Note that only site admin will be able to add images to the new custom page, since only admin has access to the admin panel.