Firstly, (and just for clarity for others who might read this), we aren't only discussing making an image physically smaller or larger - i.e altering its "pixel dimensions".
Sometimes, doing just that might be enough to reduce it down to within UK Fungi's maximum allowed file-size.
(i.e.: - the image has reduced overall pixel dimensions, and consequently has a smaller file-size).
However, that (usually) only works when the original image doesn't greatly exceed UK Fungi's maximum allowance.
It's a bit "trial and error", because it depends on your camera settings etc. etc.
It is worth noting that resizing as above does not, ( to all intents and purposes), affect image quality. - The smaller image will appear to have just the same image quality that the original larger image had.
However, if the original image "file-size" is considerably greater than UK Fungi's maximum allowed size, then you will probably need to do the above, and then "compress" or "resample" the image too.
Compressing/resampling can have a far greater effect on image file-size, because you are playing with the image quality.
Image compression is a trade-off between a much smaller file-size, and the "quality" (i.e. how the image looks on screen or in print).
But if you keep the compression within reasonable boundaries, loss of image quality isn't generally noticeable.
You say that resizing photos is too complicated for you?
There are many different software options for doing so, each with its own slight differences. (For example, I use PaintShopPro).
The process really is very straightforward and quick to do, once you have done it a few times.
I don't know if you've seen it, but this post by Adam, describes a step by step procedure for both re-sizing and compressing images, and where you can get the software download needed. This is a good as anything if you don't already have image processing software: -
Image resizing comes in handy for all sorts of reasons, so it's worth the initial effort to learn how it's done.
I really do recommend that you give it a proper try-out.
As a pretty keen photographer, I'd be well and truly lost if I wasn't able to do it.
Without a word of a lie, after getting them from the camera and onto the computer, every single image that I keep, (and I've got many thousands), will have gone through that process (along with possibly several others, such as: - sharpening / colour balance corrections / colour saturation / brightness / etc. etc.