Ok, so first of all, I know that what im gonna say might seem obvious for veterans. But as you can see this step-by-step very simple tutorial isn't for veterans (but I love u anyway)
Now let's get to the point

Recently I've played some FpsC games, older and newer ones, and one thing that is in my head now, is that almost nobody is paying attention to textures. I mean optimization-wise. Every developer wants his game to run as smooth as possible, and we are doing lots of level-design tricks to get decent fps, but let's don't forget about textures

If you go into folders with your free stuff or even model packs, you will see that some of the props have ridiculously big and heavy textures. For example - apples, slices of pizza, ashtrays, and other very small stuff from Apocalypse pack have 1024x1024 textures. And that is for an apple or slice of pizza, you know? Most of the time developers put a lot of this small stuff in games thinking that if its so small it cant be that bad for fps right? Wrong

You put some of these small stuff in your room, and nobody would ever notice that your sweet banana on your table have some super high resolution tex, but everybody would notice long loading times and low fps

Sometimes few boxes and piles of rubbish can take more memory than all enemies in your game, and that is ridiculous

So what is the solution?
1. Download gimp - its free and now it have dds plugin from the start, so its perfect for the job.
2. Now open some texture that you're using in your game, and check-out what it is. Is it 2048x2048 barrel from castle pack? Perfect

Let's open it, than press SHIFT+S on your keyboard, and now type resolution you want - Im using 512x512 for props like barrels, or boxes, 256x256 or sometimes even 128x128 (yes that small) for some rubble like small bricks or fruits; and for big things I go with 1024x1024 - most of the time is more than enough even for whole buildings
3. Ok, you adjusted scale of your texture, what to do now? Let's click file-->Export as--> now if you don't change the name it would just overwrite old texture (if your texture isn't in dds format, then change it to dds) ---> click export --> click replace (if you want to overwrite, if not - change name) -->now chose compression BC1/DXT1 (its the lightest, so the best for us) --> click export and job is done
Its best to export textures in dds format, because FpsC always change every other format to dds anyway, so it must be the best for the engine.
And for example - look at this picture: on the left side we have standard barrel from castle pack

It has 2048x2048 texture which weigh 2731kb (yes 2,7mb, and that is more than some character textures)
On the right side we have the same barrel, but now it has 512x512 texture

Diffrence in looks is unnoticable. Weight? 128kb

Yes, over 20 times less