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Need to add many objects to an image using Gimp

I would like the ability to come back that image and update any element which I add today. Does each event go in its own layer?

Or, is there another way to be able to edit, for example, a line? I may want to reduce the line width a month from now...

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the easiest way to accomplish this is to open the initial image, then File menu ―> Save as "my new file name"

this will save the file in the native GIMP file format my new file name.xcf

now, for each element you want to add, create a new layer with a TRANSPARENT background and add the element (line, or whatever)

if the elements are already in a file, then use File menu ―> Open to open it separately and modify it by :
1 - adding a transparent background by selecting Layer menu ―> Add Alpha Channel
2 - if necessary, erase the parts of the image you don't need or want
3 - select  File menu ―> Export as and (MY preference) Element file.TIF (but DO NOT select Element file.JPG, since the .JPG format does not support transparency)

now you return to my new file name.xcf and use File menu ―> Open as layer and navigate to Element file.TIF, select it and click OK

ALWAYS save this file as .xcf...

as previously mentioned, you can easily toggle each layer on/off using the Eye icon.

please note that there is a difference between SAVE and EXPORT in gimp - SAVE is only for .xcf files EXPORT is for all other file formats...

if you need more clarification, please post.
AHA! - i found an interesting article on layers in GIMP:
Layers are a very important component in doing almost anything in GIMP or an other image editing program. When you are first learning about layers, the easiest way to think of them is to see each layer as a transparent glass panel. In other words every layer acts an independent transparent glass panel. The layers stack up over each other just like a stack of transparent glass panels. What that means is that if there are transparent areas in the layer you see all the way through the layers from top to bottom just like looking down a stack of glass panels.

Topics covered:
  • The GIMP .XCF File Format
  • Layer Transparency
  • The Base Layer
  • The Layers Panel
  • Layer Modes
  • Layer Adjustments
  • Adjusting Brightness or Contrast with Layer Modes
curiouswebsterSoftware Engineer