Animated Graphics Using GDI+?

First part of a huge block will contain some text. That block is not always visible. it will be visible when the user is inputing a control. It is drawn in a bitmap image which will later be a part of the final image of the form.The second part consist of several small bitmap images most probably will contain a letter. These images will not change but there position will keep changing by time. These images will be free to move even beneath the huge block (If visible).I used a timer with 1 sec. interval to redraw form, but when the block is visible it keeps flickering because the timer clear the form and redraw the block.

I tried to use different container for the block like the PictureBox but when the small image goes beneath the block it just disappear.The block color code is "Color.FromArgb(100, Color.Black)" I tried lower Alpha components and that didn't make any difference.My explanation is that the background of the picturebox is not transparent. If I'm right how to make the picture box transparent?