A radiation-sensitive composition includes a radically polymerizable component and an iodonium borate initiator composition capable of generating radicals sufficient to initiate polymerization of the free radically polymerizable component upon exposure to imaging radiation. The iodonium borate composition includes a particular diaryliodonium borate compound having organic substituents to provide a sum of at least 6 carbon atoms on the iodonium cation phenyl rings. This composition can be applied to a suitable substrate to provide a negative-working imageable element with improved digital speed and good shelf life and that can be imaged to provide lithographic printing plates. The imaged elements can be developed either on-press or off-press using alkaline developers.