Combining and Organising Images
Layers
A layered document stacks images like acetate sheets: the top layer covers those below wherever it is opaque, and transparency lets lower layers show through. Opacity fades a whole layer; blend modes change how it interacts (Multiply darkens, Screen lightens, Overlay boosts contrast). Layer masks hide and reveal areas non-destructively - the pixels are still there, unlike erasing. Groups keep related layers tidy; smart objects preserve original data through repeated transforms.
Combining
Copying a selection and pasting into another open document normally creates a new layer there. Watch for a resolution mismatch: a selection from a 300 ppi document pasted into a 72 ppi document can appear far too large, because the pixel count is reinterpreted at the target's ppi. Use transform tools to scale, rotate and flip; use alignment and distribution to line elements up precisely.
Flattening and saving
Flattening merges all layers into one: smaller files, but no transparency and no future editing. Save a layered master (PSD or layered TIFF) and export a flattened copy for delivery. Saving straight to JPEG throws away layers.
Organising
Use a consistent folder hierarchy and file-naming convention - no spaces, ISO dates, version numbers, for example poster_2024-06-01_v02.psd. Embedded metadata (EXIF capture data, IPTC keywords, captions, ratings) makes images searchable in cataloguing software; use tags and batch rename for large sets, and always keep a backup copy.
Trap: assuming metadata survives every export - some tools strip it - and assuming layer order does not matter when a top layer hides everything beneath.