The development of long-term tumor imaging is of great importance for effective theranostic systems. In this study, a temperature-responsive poly(ether amine) with a phase transition temperature around 43 °C was used to encapsulate indocyanine green (ICG), which is a near infrared fluorescent and photothermal agent. Upon photothermal treatment, the polymeric nanoparticles underwent an increase in size from the nano- to the microscale. The prepared ICG-loaded PEAs exhibited significant stability against photobleaching and excellent NIR imaging ability. The increase in particle size resulted in the accumulation and retention of nanoparticles at the tumor site upon photothermal treatment. The accumulation of nanoparticles facilitated long-term imaging of the tumor for up to 30 days after one injection. This study highlights the potential of the rational design of polymer nanoparticles for bioimaging and diagnostics.