One of the most difficult things is to get the exposure right. Many insects have shiny bits that reflect the light, and flowers can have strong colours which easily blow the red channel. It's usually better to underexpose and then boost shadows - that also means you can use a faster shutter speed.
Resizing etc...
A JPEG is compressed so the filesize will be less than the size in RAM when displayed. If it's an 8-bit image (i.e. each channel - red, green, blue - uses 8 bits of data) there will be 3 x 8 = 24 bits of data for each pixel. If there are 10 million pixels, all these will use 240,000,000 bits (240 Megabits or Mb). There are 8 bits in a byte, so this image will use 240/8 = 30 Megabytes (MB).
So an 8-bit uncompressed TIFF from a 10 Megapixel camera will have a filesize of 30MB. A JPEG will be smaller but will still occupy 30MB of RAM when uncompressed and displayed.
When resizing, just decide on the number of pixels you want to display in the longest dimension. Use your favourite image editor, enter this value in the Resize dialogue, select bicubic interpolation (that is usually the default), make sure you tick the box to maintain aspect ratio, and away you go.
