Hi Bas!
For rendering images, it does not matter whether the image uses transparent pixels or not as the engine does not know how your image is set up (and processing every image before rendering to check for transparency would also affect performance). Each pixel of the image will take up space in memory, which is why it is the developers job to prepare images correctly for the best possible performance.
This is also the reason why using TexturePacker to create Spritesheets improves the performance when using Sprite Animations as memory usage is then optimized.
Here’s a pretty nice article about precious graphics memory (although it’s a bit older and a little tailored to Construct2, but the main principles still apply): Remember not to waste your memory
Best,
Günther