GIMP stands for GNU Image Manipulation Program, an open-source image editing program. Download it for free at gimp.org/downloads. Using GIMP, you can edit each frame of your GIF, customize the speed of your GIF, and save it in an optimized format that will be quicker to load. Go to File → Open in the top menu and select an image saved on your computer. If you'd like to draw your own GIF from scratch, use File → New instead. If you are using an existing GIMP file with multiple layers, use the Image → Flatten Image command to merge them all into one layer. This layer will be one frame of the animation. If you already have several images to turn into a GIF (such as a series of screenshots), open them using File → Open as Layers. If you only have one image and wish to edit the animation yourself, use the Duplicate Layers function in the "Layers" window on the right of the screen. You can do this by right-clicking the image icon and selecting Duplicate Layer, or by selecting the icon and clicking the icon that looks like two stacked photographs.  Each layer will be one frame of the GIF. The image on the bottom of the list will appear first, then the images above it. Drag them around to change the order. Every image must be the same size, or the larger ones will be cropped when the GIF is saved. If you plan to edit the images or add text to them, you'll need to hide all layers higher up on the list than the one you're editing, or you won't be able to see your work. There are two ways to accomplish this, both found in the "Layers" window:  Click the "eye" icon next to a layer to hide it. Click the same spot again when you're ready to show it again.  Or select a layer and adjust the Opacity bar near the top of the Layers window. Low Opacity makes the layer more transparent. This is helpful if you're adding text or other additions to multiple frames, so you can line them up with each other. You can learn about many GIMP editing features if you like, or just use these basic techniques. Select the image you wish to edit...

TL;DR in Vietnamese:
Tải GIMP. Mở tập tin ảnh muốn chuyển thành ảnh động. Thêm ảnh. Ẩn các lớp phía trên để chỉnh sửa lớp phía dưới (không bắt buộc). Chính sửa khung hình (không bắt buộc). Xem lại hoạt ảnh. Điều chỉnh thời gian. Tối ưu hóa hoạt ảnh để chạy mượt hơn. Xuất tập tin dưới định dạng Ảnh Động. Thiết lập tùy chọn và hoàn thành việc xuất tập tin.