Those are some really good tips for optimization SimonForsman, I'll try them out as soon as I can. I'm guessing this means making a sprite sheet for animation is better than having a sequence of PNG images? (Even if the sprite sheets are as big as 2000x2000 ?)
About resolutions, so you're saying the way I'm doing it is working the way it should? And that I should append a function to scale the graphics using glScale? (Also, shouldn't the graphics get larger as we increase the resolution?)
No, increasing resolution means you get more pixels on screen, if your resolution is 100x100 (very low resolution) and you got an image that is 50x50 that image will occupy 25% of the screen (or window), if you increase the resolution to 200x200 then the 50x50 image will only occupy 6.25% of the screen/window (Thus if you're using fullscreen then increasing resolution will make the graphics appear smaller, if you're using a non fullscreen window then the graphics will keep the same size relative to the monitor but not relative to the window)
If what you want to do is to have objects always occupy the same portion of the screen so that they become larger when you make the window larger then the easiest way is to pass a fixed width and height to your projection matrix (glOrtho doesn't have to take the width and height of the window as parameters, you can pass it for example 800x600 all the time and it will scale the graphics for you so that a 400x300 object always covers 25% of the window) (This will however cause the graphics to stretch if you change the aspect ratio)
Looking at your code again i think the resolution problem you have is simply that you're not using a fullscreen window (And thus its not really a problem as it is working as intended), (If you change the resolution of a normal window you are really changing its size, the actual resolution will still be whatever the desktop resolution is), You shouldn't have to use glScale unless you want to scale the graphics (scaling graphics is useful if you want two objects to use the same mesh but have different sizes).
a 2000x2000 sprite sheet shouldn't be a huge problem for modern graphicscards, older ones however will struggle though:
1) non power of two textures can be slower on old hardware (a 2048x2048 texture will often be faster than a 2000x2000 texture) and square textures are often faster than rectangular textures. newer hardware are better at dealing with odd texture sizes but can still take a serious performance hit.
2) a 2000x2000 texture might be too large for older GPUs to render using hardware acceleration or even at all . (Most new consumer cards will happily handle 4096x4096 while professional cards can often do 8192x8192 textures without problems) , you can create a proxy texture using GL_PROXY_TEXTURE_2D to see if the OpenGL implementation can use a texture of a given size, (For proxy textures the width and height will be set to 0 if the driver is unable to deal with it), there is no guarantee that it will be hardware accelerated though).
Also, you can have a sequence of png images and combine them into one or more larger textures on loadtime if you want. (This can allow you to make optimized texture atlases for each level that only includes the images you need for that specific level, or you can group the most common objects for a level into one atlas).
If for example you use sprite animations where each object always is on the same frame (so that when objectX is on frame1 objectY will also always be on frame1) it could be beneficial to keep all first frames in one texture, all second frames in another, etc (That way you only have to rebind the texture once at the start of each frame and then possibly once more to draw all non animated objects)