To write text, something like this:
Image newImg=Image.createImage(capturedImage.getWidth(),capturedImage.getHeight());
Graphics g=newImg.getGraphics();
g.drawImage(capturedImage,0,0,0);
g.drawString("string",10,10,0);
To write to memory card, read this (requires that phone supports fileconnection api jsr-75):
http://www.forum.nokia.com/document/Java_ME_Developers_Library/GUID-D69FC49D-783E-45CE-80D4-7A9F3EE08B2A/javax/microedition/io/file/FileConnection.html