Art Deco is a modern, stylised kind of look that became popular in the 1920s. It is a style that was picked up in architecture, interior design and graphic design, so you might find a room that has an Art Deco look, and you also might find a poster or post card designed using Art Deco style graphics. GIMP is a free image editing software that you can use to create all sorts of graphic projects, including a poster, postcard or other document containing Art Deco elements.
Choose a colour palette that has art deco colours. Art deco colour palettes often have several shades of the same bright colour, such as turquoise or purple, though more muted colours of beige or grey are also common. The colour palette can be all shades of the same colour, or you can also add one contrasting colour to use as an accent colour. It's a good idea to identify the colours you will use before you open GIMP and get started with your project.
Look at art deco items for inspiration (see Resources). Concentrate on the shapes and lines used in these pieces. Art Deco designs often have overlapping rectangles that look somewhat like the steps on a set of stairs, or curves that start out at the bottom as vertical parallel lines and curve outwards in both directions as they go up, ending in a rounded design like a stylised fan or peacock's tail.
Open GIMP, create a new image, and use the paint bucket tool to fill the background with one of the colours from your colour palette. Then make a new layer. Use the rectangle selection tool to create a rectangular selection slightly smaller than the outside dimensions of your image, and fill the selection with a colour that is slightly darker or slightly lighter than the background colour.
Repeat by making sever concentric rectangles, each a shade that is either darker or lighter than the next one. This can be the frame for your Art Deco piece.
Look for clip art or an icon that has an art deco look to it, such as those at Veer.com (see Resources). Open the clip art image in GIMP and use "Scale Image" from the "Image" menu to make it the right size for your project. Then copy it and use "Paste as new layer" from the "Edit" menu to add it to your image. Use the Move tool from the toolbox (it looks like a 4-headed arrow) to move the clip art to the desired location.
Use the Text tool to add text in a font that has an Art Deco feel to it, such as Broadway, Bernhard Fashion or Gallia. Choose the contrasting colour from your colour palette for the font colour, or choose a shade that is very much lighter or darker than the area behind it, so it stands out and is readable.
Save the image as a GIMP file, using the XCF format. This preserves the layers and gives you the opportunity to edit it later if you need to do so. Then save it again in a format that will allow you to use it as you need to. For example, if it will be used on the Web or inserted as an image into another document, save it as a Jpeg or PNG file. Or if you will have it printed, check with the printer as to what type of image or document file they require, such as a PDF or TIFF.